1. Field of the Invention
The present invention relates to an optical member for photolithography made of a calcium fluoride crystal, and a photolithography apparatus using this optical member; and, more specifically, to an optical member made of a calcium fluoride crystal which is employed in a stepper or scanner which is a photolithography apparatus using, for its exposure light source, an F2 laser (157 nm), an ArCl excimer laser (175 nm), a Kr2 excimer laser (146 nm), an Ar2 excimer laser (126 nm), a solid-state laser utilizing a nonlinear optical effect, or other ultraviolet rays, and a photolithography apparatus using this optical member.
2. Related Background Art
In recent VLSI (Very Large Scale Integration), density and function have become higher, thus requiring finer processing techniques on a wafer. As a method of its processing, one based on photolithography is carried out in general. At present, the exposure wavelength is becoming shorter, and a stepper using ArF excimer laser light (having a wavelength of 193 nm) is expected to appear soon. Since typical optical materials absorb light having a wavelength not longer than 193 nm, materials employable in optical members for photolithography are quite limited. While specific examples thereof include lithium fluoride crystals, magnesium fluoride crystals, calcium fluoride crystals, barium fluoride crystals, strontium fluoride crystals, and silica glass, optical members are often designed by use of two kinds of materials constituted by a calcium fluoride crystal and silica glass in general.
Also, attempts are being made to further shorten the wavelength of the light source in order to effect finer processing, and demands for photolithography utilizing vacuum ultraviolet rays such as F2 laser light (having a wavelength of 157 nm) have been increasing in recent years. Since silica glass exhibits a low transmission property with respect to such light, however, it is not considered to be suitably usable as the material for optical members. Therefore, optical members made of calcium fluoride crystals have been under development.
Conventionally, calcium fluoride crystals have been made by such methods as Bridgman-Stockbarger method (crucible descending method), Czochralski method, and the like. Among others, Bridgman-Stockbarger method is in wide use since it is advantageous in that crystal growth is carried out while being kept from contamination from the air and in that it can make a large crystal. In this method, a fluorinating agent (e.g., lead fluoride or the like) known as scavenger is added to synthesized powders of calcium fluoride whose amount of impurities is low, the resulting mixture is sealed in a crucible and heated in a vacuum so as to be melted, and then the crucible is descended at a velocity of 0.5 to 5 mm/hour so as to effect crystal growth. Thus obtained calcium fluoride crystal is subjected to a heat treatment for eliminating thermal stress, and then is cut and processed, so as to become a material for an optical member.
From thus obtained material, prisms, mirrors, and various lenses are manufactured as optical members for photolithography. These optical members are subjected to optical grinding with a required precision, and are formed with a required coating.
If a member used in an optical system of a photolithography apparatus absorbs light, then its influence on the whole optical system is strong even when the amount of absorption is very little. Specifically, there occurs such a problem that not only its throughput (light transmission property of the whole optical system) is unfavorable, but also light absorption causes the temperature of optical members (lenses) to rise, thereby deteriorating imaging performances. Therefore, for establishing a photolithography technique utilizing vacuum ultraviolet rays, it is very important that slight absorption in optical members be suppressed. In the optical members made of calcium fluoride crystals manufactured by conventional methods, however, it has been difficult to obtain desired optical performances.
Also, the above-mentioned optical members are required to have a high durability, i.e., to be kept from deteriorating their transmittance when irradiated with light. Japanese Patent Application Laid-Open No. HEI 7-281001 discloses, as a fluorite for ultraviolet optics employed in ultraviolet optical systems, one whose internal transmittance is at least 90.0% per 10 mm when irradiated with 104 to 107 shots of laser light at 50 to 500 Hz having an energy density of 50 to 500 mJ/cm2/pulse. When vacuum ultraviolet light of 185 nm or under such as F2 laser light is utilized, however, the energy density of light emitted from the light source is 50 mJ/cm2/pulse or less, and the energy density of light irradiating a member distanced from the light source or a member used in a projection optical system is 0.5 mJ/cm2/pulse or less, which is much lower. Therefore, optical members are required to maintain their internal transmittance at a higher level with respect to light having such a low energy density, whereas materials exhibiting an internal transmittance of 90.0% per 10 mm after light irradiation are insufficient as those for optical members.
One of the reasons why optical members having desired optical performances have been hard to develop as mentioned above is the lack of means for measuring their optical characteristics precisely.
For example, Japanese Patent Application Laid-Open No. HEI 9-255328 discloses a fluorite whose internal transmittance with respect to light having a wavelength of 135 nm is 70% or greater, whereas this internal transmittance is measured for a sample having a substrate thickness of 20 mm by use of a vacuum ultraviolet spectrophotometer. In such a typical measuring method, however, though it can be seen that the internal transmittance is about 70%, for example, it is difficult to carry out measurement with such a high accuracy as to determine whether it is 70.1% or 70.3%.
Therefore, it is an object of the present invention to provide an optical member suitably employable in a photolithography apparatus having an optical system which uses light having a specific wavelength of 185 nm or shorter as a light source, and a photolithography apparatus using the same.
The inventors have studied a method of accurately measuring the internal transmittance of an optical member exhibiting only slight optical absorption by use of a typical spectrophotometer for a vacuum ultraviolet region. As a result of repeated diligent studies using this measuring method, the inventors have accomplished the present invention concerning an optical member suitably employable in an optical system which uses light having a specific wavelength of 185 nm or shorter as a light-source, and a photolithography apparatus using the same.
Namely, the optical member in accordance with the present invention is an optical member for photolithography made of a calcium fluoride crystal exhibiting an internal transmittance of 99.5%/cm or greater with respect to light having a specific wavelength of 185 nm or shorter.
Also, the photolithography apparatus in accordance with the present invention is a photolithography apparatus comprising an exposure light source for emitting light having a wavelength of 185 nm or shorter, a mask formed with an original image of a pattern, an irradiation optical system for irradiating the mask with the light emitted from the light source, a projection optical system for projecting onto a photosensitive substrate a pattern image projected from the mask, and an alignment system for aligning the mask and the photosensitive substrate with each other;
wherein at least a part of optical members constituting the irradiation optical system and projection optical system is made of a calcium fluoride crystal exhibiting an internal transmittance of 99.5%/cm or greater with respect to light having a specific wavelength of 185 nm or shorter.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.